1. Field of the Invention
The present invention relates generally to laser oscillators and, more particularly, to laser oscillators having a negative spherical aberration.
2. Discussion of the Related Art
High power solid state lasers based on rod geometry use laser oscillators containing a sequence of elements, such as lenses, mirrors, or Gaussian apertures. These components transform beam characteristics (such as scale size and wavefront curvature) each time the beam encounters them in the course of a round trip. A stable laser oscillator is one for which a particular beam can start out and return to its initial position unchanged in scale and curvature from the beam which started out. A maximum of one such beam is possible for a given laser oscillator.
If the laser oscillator includes a lens having a variable power (inverse of focal length), then a small change in the lens power will produce a change in the scale size of the beam that is supported by the laser oscillator. This change can be to increase the beam scale with increasing lens power. Laser oscillators which increase the beam scale with lens power are said to have a xe2x80x9cpositive slope,xe2x80x9d and those which decrease beam scale with lens power are said to have a xe2x80x9cnegative slope.xe2x80x9d
Prior art laser oscillators that achieve high beam quality and efficiency tend to have a positive slope. These laser oscillators, which are designed for high beam quality and efficiency, achieve large beam scale size in the rod by operation near an upper stability limit where scale size becomes infinite. At the upper limit (in terms of rod focal power), the laser oscillator slope is positive and, at the lower limit, it is negative. Operation at the upper limit allows the laser pump power to be turned down (since rod focal power is proportional to pump power) and still be stable, which is a convenience for alignment of the laser resonator. Further, in these laser oscillators with positive slopes, degradation due to aging does not cause the laser resonator to go unstable. Such high power solid state lasers, however, suffer from large spherical aberration, lensing, and strain birefringence.
On the other hand, current laser oscillators that operate near the lower limit of beam stability (i.e., laser oscillators with a negative slope) have higher power thresholds. These laser oscillators cannot, however, be operated at lower powers and aging tends to make the laser oscillator unstable. Thus, there exists a need for a solid state laser operating near the lower limit of stability that outputs high power and high quality beams.
The present invention is a laser oscillator having a reflective mirror and an output coupler mirror. Two gain media are disposed between the reflective mirror and the output coupler mirror. The gain media are pumped with energy to produce laser oscillation between the reflective mirror and the output coupler mirror. Each of the gain media has a focal strength during operation that is formed by thermal and mechanical stress. The laser oscillator also includes a lens system that is disposed between the two gain media. The lens system has a focal strength about equal to or greater than a product of the focal strengths of the two gain media. Also included is a polarization rotator disposed between the two gain media.
The lens system is a negative power lens system that forces the laser oscillator to have a negative slope. The reflective mirror is preferably graded. Thus, the laser oscillator is able to achieve a high output with spherical aberration.
The above summary of the present invention is not intended to represent each embodiment or every aspect of the present invention. This is the purpose of the Figures and the detailed description which follow.